The present invention relates to a device that controls power to an electric load. More specifically, the present invention relates to sensors for detecting the presence of a person in an area and activating and controlling lights or other apparatus accordingly.
There are a variety of sensors on the market that detect the presence of a person in a room for the purpose of automatically turning lights ON and OFF. Most often these sensors detect a person's presence by detecting either movements of the person or his or her body heat. Movement is commonly detected by using an ultrasonic transmitter and receiver that are mounted at a position in a room that is most likely to detect movement. The ultrasonic transmitter transmits an ultrasonic signal that reflects off objects in the room and is detected by the ultrasonic receiver. An examination of the frequency of the received waves can be used to detect movement by the Doppler effect. Because of the ability of ultrasonic waves to bounce off objects, ultrasonic motion detectors can suffer from the disadvantage that movement which occurs outside of a room or desired area can be detected by the sensors. Thus, ultrasonic motion detectors have a tendency to switch lights ON when the lights are not actually needed.
Another method of detecting the presence of people is by using an infrared sensor that detects the heat given off by a person in the room. Such infrared sensors detect a rapid change in heat, such as when a person enters a room, and are mounted at a position in the room in which they are likely to detect a person. Because infrared sensors only detect rapid changes in heat, they often correctly distinguish between slow and constant heat increases brought on by a device such as a heating element, computer, or coffee maker in the vicinity of the sensor. However, when heat sources that rapidly change temperature such as tungsten lamps are present, infrared sensors often falsely indicate the presence of a person. Additionally, infrared sensors may not be able to detect a person who is not in the immediate vicinity of the sensor or is blocked by a chair, bookcase, or other object.
The level of sensitivity of infrared sensors is typically adjustable. However, adjusting a sensor so that it is more likely to detect a person at a distance means the sensor will also be more likely to falsely indicate the presence of a person if some other heat source is present. Accordingly, since prior art sensors only allow for the sensitivity of the sensor to be adjusted, false readings are likely to occur in certain situations.
The above problems of correctly determining the presence of a person are greatly exacerbated if a workroom is large and commonly has only one person working at one of two or more different locations in the room.
There are also sensors on the market that adjust the brightness of lights in a room or turn the lights ON and OFF depending on the level of ambient light detected by the sensor. These sensors are subject to the same difficulties and inefficiencies as the prior art sensors detecting the presence of a person in that they too are mounted in a single position, usually near the lighting source. However, if a person frequently works far from the location of the sensor, lights that are needed in order to perform work tasks may be switched OFF or dimmed by the sensor. Some ambient light sensors are adjustable to turn lights ON and OFF at different levels, but the adjustment mechanism is on the sensor or at a power switching location requiring a person to move each time an adjustment in the sensor's sensitivity is to be made.